What you’ll need
Step by step
Lay a true first course
The first course sets the whole wall. Bed it dead level and to line on a conventional levelling mortar — everything above depends on this course being right.
Mix BJM to a smooth paste
Add SUMO BJM powder to clean water and mix with a paddle to a lump-free, creamy but firm paste. Let it stand briefly and remix. Mix only what you can lay before it starts to stiffen.
Prepare the blocks
Brush dust off the block faces. Lightly dampen very dry blocks so they do not rob water from the thin mortar bed, but do not soak them.
Comb on a thin bed
Apply BJM to the bed and vertical joint face with the notched applicator to leave an even 2–3 mm ribbed layer. Thin and even is the goal — this is not a thick levelling bed.
Lay, tap and check
Place each block, tap down with a rubber mallet to the string line, and check level and plumb as you go. The thin bed grabs quickly, so line the block up as you set it.
Keep courses true
Check level, plumb and line every few courses. Clean off squeeze-out before it sets, and let the wall gain strength before loading it.
What to avoid
It is a 2–3 mm thin-bed adhesive. A thick bed wastes material and weakens the joint. Level the first course conventionally instead.
Thin-bed walls cannot hide a bad start. Lay the first course dead level and to line.
Soaked blocks and a sloppy mix both weaken the joint. Lightly dampen very dry blocks only.
BJM stiffens as it stands. Mix batches you can lay in the working time.